The present invention relates to a combustion method for an industrial furnace. It also relates to a furnace suitable for implementing such a method.
The heating power distribution on a given furnace surface, the reduction of the quantity of nitrogen oxides produced, and the stability of the combustion flame(s) generated in the furnace, are among the main challenges in combustion furnace technology.
In fact, the energy efficiency and the profitability of an industrial combustion furnace are higher for large capacity furnaces. This is why the surface to be heated may be large. This is generally the upper surface of a charge of raw materials or melt contained in a chamber. It is accordingly difficult to distribute the heating power delivered by the combustion flame(s) substantially uniformly over the whole surface, to prevent the formation of colder zones that would be harmful to the melt or to the subsequent method for treatment thereof. For this purpose, a plurality of burners is known to be arranged in a furnace, at predefined locations above the chamber. In particular, two burners can be placed in parallel to one another, with respective horizontal flames directed in the same direction. Another alternative is to position the burners in opposing pairs, with the respective flames directed at one another within each pair.
Furthermore, the quantity of nitrogen oxides (Nox) produced in a combustion flame depends on the local oxygen and nitrogen concentrations, denoted [O2] and [N2]. In particular, an evaluation of the quantity of thermally produced nitric oxide (denoted [NO]th) is given by the following equation:
                                          ⅆ                                          [                NO                ]                            th                                            ⅆ            t                          ≈                              k                          exp              ⁡                              (                                                      E                    a                                    RT                                )                                              ·                                                    [                                  O                  2                                ]                                            1                /                2                                      ⁡                          [                              N                2                            ]                                                          (        1        )            where k is a numerical constant, exp denotes the exponential function, Ea is a positive activation energy, R denotes the ideal gas constant and T is the local temperature.
In order to reduce the quantity of thermally produced nitric oxide, use of a substantially nitrogen-free oxidizer is known. Thus, an oxygen-enriched oxidizer is used instead of air. However, the reduction of the resulting nitrogen oxides is insufficient to meet the regulations in force.
To further reduce the quantity of nitrogen oxides produced, it is also known, particularly from U.S. Pat. No. 5,522,721 and EP 0 524 880, to cyclically vary the oxidizer flow rate and/or the fuel flow rate fed to the flame. The ratio between the local instantaneous concentrations of oxygen and fuel in the flame is accordingly different from the stoichiometry of the combustion reaction. The local temperature is consequently lower and, according to the equation (1), this causes a further reduction of the quantity of thermally produced nitric oxide. However, the flow rate variation parameters, such as the amplitude, the frequency and the phase of the variations of the flow rates, are difficult to adjust to obtain a satisfactory heating efficiency and a low release of carbon monoxide (CO). In fact, carbon monoxide is toxic and pollutant, and is generated by incomplete combustion when the local instantaneous oxygen concentration in the mixture is too low compared to the local instantaneous concentration of fuel.
Another way to obtain a further reduction of the quantity of nitrogen oxides produced consists in injecting a main part of the oxidizer and the fuel at two locations of the furnace separated from one another by a relatively long distance. A combustion carried out under these conditions is called “staged” (see for example EP 0 748 981). A small part of the oxidizer is also injected close to the fuel outlet to stabilize the combustion conditions. The main part of the oxidizer and the fuel are then mixed progressively in the spread volume where the jets overlap. In this way, a gap effect is also obtained, between the ratio of the local fuel and oxidizer concentrations on the one hand, and the stoichiometry of the combustion reaction on the other. Furthermore, this stoichiometric gap effect is superimposed on a dilution effect. The local temperature, and consequently the quantity of nitric oxide, are thereby also reduced. However, in this staged combustion configuration, the position of the flame in the vertical direction is particularly unstable. The efficiency of heating of the charged material is accordingly reduced and the roof refractories may be damaged.
It is therefore an object of the present invention to propose a combustion method which does not have the abovementioned drawbacks, or in which these drawbacks are reduced.